A Williamite is a follower of King William III of England who deposed King James II in the Glorious Revolution. William, the Stadtholder of the Dutch Republic, replaced James with the support of English Whigs.
One of William's aims was to ensure England's entry into his League of Augsburg against France in the Nine Years War. For Williamites in England, Scotland and Ireland, William was seen as the guarantor of civil and religious liberty and the Protestant monarchy against Catholic absolutism.
The term "Williamite" is also commonly used to refer to William's multi-national army in Ireland during the Williamite War in Ireland, 1689-91. In Ireland itself, William was supported by Protestant settlers and opposed by the native Irish catholic Jacobites who supported James. He is still depicted in the iconography of the Orange Order, whose name comes from William's dynasty, the House of Orange-Nassau.
"Williamite" is sometimes applied to Late Stuart country house architecture built c 1690 - 1710 in the conservative classicising English tradition that had been established under Charles II by Hugh May and Sir Christopher Wren, of which Belton House, Lincolnshire, and, formerly Stoke Edith, Herefordshire are typical examples. Such compact houses do not fit easily within the conventions of English baroque architecture.
The "Williamite Purple Star" is up to the present part of the flag of the Orange Order in North Ireland.